Personally being this my first Jam ever I felt relived when they said we could start working on the game asap (as it usually comes down to fewer time frames), but I totally get what you are saying, I ended up working 9 days straight from scratch and the last day I almost gave up trying to make some music for it, then when you finally post your game and take a look at the others, there are some games (like the one Pixel Team mentioned) that look like a two months old project.
I mean I know re-using assets and some code it's allowed, but re-skining a game or just finishing an old ongoing project for a jam, just doesn't feels right.
But as we all know, its th Rules fault.
I hope this experience makes future jams even better, I certainly enjoyed working on a rush for this one~